The quantum yield of phytoplankton was found unchanged, increased or decreased with increasing depth under the light saturating depth in the in situ experiments,and using three species of marine micro-algae. The increase ordecrease in quartum yield that deviate from the theoretical model may be attributed to"the factors such as light-shade adaptation of phytoplankton.The relation ships of the quartum yield with photosynthetic rate,primary productivity and radiation utilization efficiency were discussed also...